The following invention relates to a method of producing defect profiles in a crystalline or crystalline-like structure, preferably in a semiconductor, during a thermal treatment in a process chamber.
During the manufacture of semiconductor components, for example based on silicon, it is known, by thermal treatment steps of the semiconductor in a suitable process gas environment, to influence the distribution of foreign atoms introduced (implanted) in the semiconductor or to influence the distribution of crystal defects. In general, the distribution of foreign atoms is essentially codetermined by the distribution of the defects.
For example, it is known from W. Lerch et al.; Mat. Res. Soc. Symp. Proc. (1998), Vol 525, pp 237-255 and D. F. Downey et al.: Mat Res. Soc. Symp. Proc. (1998), Vol 525, pp. 263-271, that by means of an oxygen-containing process gas at a constant thermal stressing of an implanted or doped silicon semiconductor, the doping profile of boron can be influenced, since the oxidation of silicon leads to an over saturation of self-interstitial atoms (EZG, a type of point defects with which Si atoms are disposed on interstitial positions), the concentration of which influences the diffusion property of the boron and thus the doping profile of boron.
With an oxygen-containing process gas environment during the thermal treatment of semiconductors, essentially only those foreign atom profiles or doping profiles can be influenced, the foreign atoms of which essentially reach a lattice position via the kick-out mechanism. In so doing, the foreign atom previously disposed in the interstitial region reaches a lattice position, whereby a silicon atom (or in general lattice atom) is given off from this lattice position into an interstitial position.
U.S. Pat. Nos. 5,401,669 and 5,403,406 describe a selective formation of defects via a nitrogen-containing process gas atmosphere, whereby these defects serve as nucleation centers for the precipitation of oxygen dissolved in silicon.
D. F. Downey et al.; Mat. Res. Soc. Symp. Proc. (1997), Vol 470, pp. 299-311 describes a reactive process gas during the rapid thermal treatment (RTP: Rapid Thermal Processing) of doped silicon wafers with regard to the doping profile for various reactive gases of different concentrations.
In the not yet published German application 199 27 962 of the applicant there is presented a method for the adjustment of the doping profile that is improved relative to D. F. Downey et al. (Mat. Res. Soc. Symp. Proc. (1997) Vol 470, pp 299-311). In this case, a semiconductor is essentially subjected to a rapid thermal treatment (in an RTP system) in a process gas atmosphere that includes a number of reactive gases in order, for example, via these reactive gases, to activate different inherent or self-point defects simultaneously and in different concentrations. In general, defect concentration and/or their spatial defect distributions, and hence also the distribution(s) of the activated doping atoms, can be controlled by the described method.
With the increasingly smaller structure sizes of semiconductor components, the requirements also increase with regard to the possibilities of the control of defect and doping profiles (here in the sense of the spatial distribution of activated foreign atoms). It is therefore the object of the present invention to further improve the above-described method.
The present invention realizes this object by the initially mentioned method, whereby a concentration and/or a density distribution of defects is controlled as a function of at least two process gases that have different compositions and each contain at least one reactive component, and whereby at least two of the process gases, essentially separated from one another, respectively act on at least two different surfaces of the substrate.
With the inventive method, for example the front and rear side of, for example, disk-shaped semiconductors, for example silicon wafers, are brought into contact with different reactive gases. This makes it possible during a thermal treatment of the wafer to control defect distributions and/or defect concentrations from both sides of the wafer, whereby as a result a maximum of possibilities are possible, especially with regard to the concentration and the spatial distribution of defects, but also with regard to the type of defects, for example point defects and/or volume defects. The inventive method can preferably be used in RTP- (Rapid Thermal Processing-), CVD- (Chemical Vapor Deposition-), RTCVD- (Rapid Thermal Chemical Vapor Deposition-) and epitaxial units, in which generally a xe2x80x9csingle wafer processingxe2x80x9d is carried out, i.e. with which respectively only one wafer is subjected to a thermal treatment. However, a plurality of wafers could also be simultaneously subjected to a thermal treatment, as is effected, for example with many epitaxial or CVD units. In such a case, a plurality of wafers (for example two to six) are essentially disposed in a single plane. As a consequence of a thermal treatment of respectively only a single wafer or only few wafers, that are essentially disposed within a plane, there exists the possibility of respectively bringing the wafer or wafers, each with front and back sides, pursuant to the inventive method in contact with essentially only one process gas, whereby the composition of the process gases is different and each includes at least one reactive component.
Pursuant to the present invention, a process gas includes at least one reactive component i.e. a component that reacts with the substrate. The reaction can be a chemical reaction of the component (for example O2) with the substrate (for example Si) to form a new substance (for example Si+O2 greater than SiO2), an adsorption (physical sorption and/or chemical sorption) of a component of the process gas, and etching (chemical reaction) or a desorption of an already present adsorbent by components of the process gas. The reaction can be effected at the substrate surface (for example, beginning of the oxidation of Si, or the oxidation of a copper layer applied to a substrate), on a portion of the substrate surface, or also within the substrate (for example the oxidation of Si of the already present silicon oxide layer). Furthermore, the reaction can also be effected on defects within the substrate, as is the case, for example, with the reduction of SiO2 by hydrogen, whereby the SiO2 is disposed on the inner surfaces of COPs (crystal originated particles). The process gas can include inert gases, i.e. gases that do not react with the wafer or the substrate. Whether a gas or a gas component behaves in an inert fashion can depend upon the process temperature, with an example for this being N2 during the processing of silicon. At temperatures of up to about 1100xc2x0 C. N2 behaves nearly inert; only at temperatures over 1100xc2x0 C. does any appreciable action with silicon in the form of a nitridation occur. Other inert gases are, for example, noble gases such as argon, helium, neon. For this connection, helium is characterized, for example, by a particularly high thermal conductivity, which can, for example, be advantageous during the rapid cooling of the substrate.
With the phrase xe2x80x9cprocess gases that are essentially separated from one anotherxe2x80x9d it is to be understood that the process gases taking part in the process at most mix with one another to such an extent that at a surface or in a region of this surface, reactions dominate that are generated by the process gas that predominantly acts upon the surface. Ideally, the process gases would be entirely separated by the substrate, so that a mixing of the process gases in the vicinity of the respective surface is not possible, and each process gas acts only on one surface of the substrate. However, the fulfillment of this ideal condition depends greatly on the technical design of the process chamber and upon the holding device of the substrate. However, the inventive method can also be carried out even if this ideal condition is not completely fulfilled, with one taking care, for example, that the process gases that act upon various surfaces of the substrate mix only minimally with one another, or mix only to such an extent that, as described above, one reaction dominates.
In the following, the term defect is intended to include zero to three-dimensional lattice defects. Zero-dimensional defects are, for example, point imperfections, or point defects such as vacancies, self-interstitial atoms (EZG or interstitials) and chemical foreign atoms that are disposed in the host or matrix lattice on interstitials or lattice locations. Depending upon whether the defects are caused by host lattice atoms or foreign atoms, one speaks of intrinsic or extrinsic point defects. If the host lattice atoms that cause the vacancies wander to the surface, there result Schottky defects; if these atoms wander to interstitial positions, one talks of Frenkel defects. An accumulation (agglomeration) of point defects can lead to higher dimensional disorders, such as, for example, displacement rings or displacement lines (one-dimensional defects), stacking errors (two-dimensional defects), or precipitates of foreign atoms (three-dimensional defects).
Further defects are, for example, grain boundaries (two-dimensional) or the already mentioned three-dimensional precipitates (for example oxygen precipitates in silicon or metal precipitates), or the nucleation centers necessary for the formation of precipitates as well as local amorphous regions that result, for example, during ion implantations or voids. The term crystalline-like should be understood to mean, for example, the transition region from crystalline to amorphous structure. A further defect is also the formation of F-centers (Farnsworth centers), such as are present, for example, in ion crystals, where an electron stops in a halogen gap in the vicinity of the adjacent cations.
Pursuant to one preferred embodiment of the invention, the two different surfaces of the substrate are disposed approximately opposite one another, and the density distributions of the defects produced by the reactive components of the process gases extend from each surface in a direction toward the respective other surface. With this embodiment, involved are in general disk-shaped substrates, for example semiconductor wafers of silicon. In this connection, the front and rear sides of the wafer are respectively brought into contact with the respectively different process gases during a thermal treatment step.
The density distributions of the defects produced by the process gasses are preferably essentially self-interstitial atoms at the one surface and vacancies at the other surface. The density distributions then extend respectively in the direction toward the respective other surface. If the defects, for example with a silicon wafer, on the front side are self-interstitial atoms, i.e. are to result on the side of the structures, or structures are already present, then for example the diffusion of boron present in the silicon is promoted in the direction of the interior of the wafer, or in general, the diffusion of foreign atoms (that use these defects from self-interstitial atoms that fall in the class of inherent point defects as xe2x80x9cdiffusion vehiclesxe2x80x9d) is promoted in the direction counter to the gradients of the density distribution of the self-interstitial atoms, or is obstructed in the direction of the gradients. The behavior is correspondingly reversed if the defects are vacancies, then the diffusion of foreign atoms in the direction of the gradients of the density distribution of the vacancies is promoted and is obstructed in the direction counter to the gradients of this density distribution. This characteristic can be advantageously exploited with the manufacture of flat zones that are implanted with foreign atoms by generating an excess of vacancies from the surface at which the implanted zones are disposed. On the side of the crystal (generally a wafer) opposite to the implanted zones, an excess of self-interstitial atoms is advantageously produced. The term excess is here understood to mean an exceeding of the defect concentration that would be established in the thermal equilibrium at the respective process temperature. By generating such defect distributions, the diffusion of foreign atoms in the direction of the interior of the crystal is essentially obstructed, as a result of which flat zones of foreign atoms can also be produced after a thermal treatment of the crystal, for example after an activation step. As a result one obtains flat PN junctions, so-called ultra-shallow-junctions.
It can furthermore be advantageous for the density distributions to respectively comprise self-interstitial atoms and vacancies that proceed from the respective surface and that differ in concentration and/or in their ratio relative to one another. Such density distributions, can, for example, be produced by suitable process gases that include not only reactive components for the production of self-interstitial atoms but also reactive components for the production of vacancies. With such process gases it is possible in particular to influence the spatial profile of activated foreign atoms in a crystal lattice, for example silicon.
Furthermore, the different process gases that act, for example, on the front side and the rear side of a wafer, can advantageously have different thermal conductivity coefficients and/or heat-absorption capacities. As a result it is possible, for example, to enhance a varying rapid cooling off or heating up of the wafers at the respective surfaces, as a consequence of which, within the wafer, there is formed from one surface to the other a temperature gradient that, for example, in turn influences the diffusion property of defects.
The present invention is advantageously utilized to process a substrate having a predefined temperature/time profile within a process chamber. As a result of the temperature/time profile, the diffusion property of the defects can be controlled significantly.
Additionally, at least one process gas can be advantageously correlated with the temperature/time profile with regard to its composition and/or its process gas pressure and/or with regard to its process gas temperature. In this connection, for example, the concentration (partial pressure) of the reactive components of a process gas, the total pressure of the process gas, but also the temperature with which the process gas flows into the process chamber and acts upon the substrate, can be correlated with the temperature/time profile.
The inventive method can also be advantageously utilized with an xe2x80x9cundopedxe2x80x9d silicon wafer, with which is to be understood such wafers that after manufacture of the wafer are not additionally doped with foreign atoms by, for example, diffusion and/or implantation and/or some other process. With such wafers the spatial defect distribution can be adjusted or established that is then used for fixing the oxygen distribution within the silicon crystal. The method can, for example, be advantageously used for establishing the precipitate distribution of oxygen.
The use of the inventive method with xe2x80x9cdopedxe2x80x9d silicon wafers can, as already indicated above, also be advantageous, for example, during the production of flat pn junctions. In this connection, doped wafers means those wafers that are additionally doped with foreign atoms, for example by diffusion and/or implantation and/or some other process. If the inventive method is used, for example, with doped silicon wafers, it is advantageous if certain defects are essentially in a surface layer between 0 and 150 xcexcm, whereby density, quantity or concentration decrease toward the interior of the substrate. An example of this is vacancies.
If in contrast the inventive method is used for the preparation of silicon wafers, it is advantageous if certain defects are essentially depleted in a surface layer between 0 and 150 xcexcm, and the defects are essentially disposed in the interior of the substrate. In this case, the defects are, for example, essentially vacancies at which, for example, oxygen precipitates can form. In so doing, there is advantageously produced at the wafer surface a precipitate-free layer of about 150 xcexcm within which structures, such as, for example, electronic circuits can be produced.
The inventive method can also be used with production wafers, i.e. wafers that already include structures that are obtained during the manufacture of integrated circuits, discrete components, or in general electronic circuits. Furthermore, the wafers or substrates can also include components from the microstructure technology.
In general, as already mentioned above, the defects can be foreign atoms. This is in particular the case with doped crystals. However, the defects can also be vacancies at which is effected, for example, a precipitate formation, so that the defects are, in effect, precipitates.
Furthermore, the defects can also be xe2x80x9ccrystal originated particlesxe2x80x9d COPs, which are controlled via the inventive process with regard to their spatial distribution and/or concentration.
Advantageously, the defects generated by at least one process gas can produce a getter layer in a layer in the vicinity of one surface, whereby the getter layer can be used, for example, for the gettering of impurities such as metals. Such a getter layer can, for example, comprise an accumulation of oxygen precipitates in silicon. As described above, such a precipitate layer can, for example, be produced via a suitable spatial vacancy distribution.
By means of the inventive method, defects such as vacancies and/or self-interstitial atoms are produced, whereby the reactive process gas preferably includes oxygen and/or nitrogen and/or an oxygen and or nitrogen-containing component. In this connection, for example one of the process gases can essentially comprise oxygen and/or an oxygen-containing gas, i.e. the fraction of other gases is, for example, less than 100,000 ppm.
For certain processes, it can, however, also be advantageous if one of the process gases is less than 50,000 ppm, preferably less than 1,000 ppm, and with many processes even less than 250 ppm, although more than 10 ppm oxygen (or in general oxygen-containing gas) and/or nitrogen (or in general nitrogen-containing gas).